1. Field of the Invention
The present invention is concerned with the preparation of sulfonyl isocyanates having the general formula EQU R--SO.sub.2 --NCO I
wherein an isocyanate group is linked directly to the SO.sub.2 group and R is a C.sub.1 - to C.sub.18 -alkyl, a phenyl, a C.sub.1 - to C.sub.18 -alkyl phenyl radical or an isocyanate group.
2. Discussion of the Prior Art
Compounds having the formula I became of importance with respect to the stabilization of organic isocyanates against decomposition and discoloration (see U.S. Pat. No. 33,330,849, DE-Pat. No. 2 030 316), and the stability increase of polyurethane prepolymers (DE-Pat. No. 1 245 590). Furthermore sulfonyl diisocyanate forms an intermediate compound for producing diacyl sulfamides which are used for stabilization and acceleration of reactive acrylate adhesives (German patent application No. P 31 37 306.2-43).
Although sulfonyl isocyanates are already known since the beginning of this century, systematic studies of this class of substances started only 50 years later. For one reason this results from the difficulty of obtaining these compounds synthetically and for another from their high reactivity. A summary of the methods of forming these compounds known in 1964 is given by H. Ulrich in Chem. Rev. 65 (3), 369-376 (1965). From the voluminous recent literature the following important general methods of obtaining sulfonyl isocyanates are available:
1. Methods of preparing aliphatic sulfonylisocyanates
1.1. Reaction of sulfonylchlorides and anhydrides with silver isocyanate: EQU CH.sub.3 --SO.sub.2 --Cl+AgNCO.fwdarw.CH.sub.3 --SO.sub.2 --NCO+AgCl PA1 O. C. Billeter, Ber. 38, 2013-2015 (1905); L. Field, P. H. Settlage, J.Am.Chem.Soc. 76, 1222-1225 (1954). PA1 1.2 Elimination of alcohols from sulfonylurethanes or their silylated analogues: ##STR1## 1.3 Heterogeneous reaction of sulfochlorides with alkali metal cyanates: ##STR2## 2. Methods of preparing aromatic sulfonylisocyanates. 2.1. Reaction of sulfonamides with phosgene in high boiling inert solvents: ##STR3## 2.2 Reaction of N-alkyl-N'-arylsulfonylureas, which are intermediates from the reaction of sulfonamides with alkyl isocyanates, with phosgene: ##STR4## 2.3 Reaction of aromatic sulfonylchlorides with alkali metal cyanates: ##STR5## 2.4 Reaction of aromatic hydrocarbons with chlorosulfonylisocyanate: ##STR6## 3. Methods of forming sulfonyldiisocyanate 3.1 Reaction of bromocyanogen with SO.sub.3 : EQU 2BrCN+2SO.sub.3 .fwdarw.OCN--SO.sub.2 --NCO+SO.sub.2 +Br.sub.2 PA1 R. Graf, DE-PS 940 351. PA1 3.2 Thermal or catalytic decomposition of chlorosulfonylisocyanate: ##STR7## 3.3 Reaction of chlorosulfonylisocyanate with silver isocyanate: EQU Cl--SO.sub.2 --NCO+AgNCO.fwdarw.OCN--SO.sub.2 --NCO+AgCl PA1 R. Appel, H. Gerber, Ber. 91, 1200 (1958). PA1 All of the above mentioned processes suffer from substantial disadvantages. The use of silverisocyanate for instance is impeded by the high price and the laborious, time-consuming and expensive regeneration of the substance while when using potassium cyanate only a small yield is obtained from the heterogeneous reaction. It is impossible to obtain a homogeneous reaction by use of a suitable solvent since the strong reactivity of the sulfonylisocyanate results, especially under the influence of high temperature, in a yield-reducing reaction of the same with the solvents in question. In reactions using phosgene it is disadvantageous that the substance is highly poisonous and thus a health hazard; furthermore the by-products have to be removed. In elimination reactions from sulfonylurethanes it is an unfavourable fact that these starting materials first have to be specially synthesized and subsequently it is not possible to regenerate the elimination products in the conversion reaction itself.
In the methods of preparing sulfonyldiisocyanate known in the art it is on the one hand the high toxicity of the rather expensive bromocyanogen which is interfering and on the other hand the fact that during the thermal decomposition of chlorosulfonylisocyanate only one moiety of the starting material is utilized, while the other moiety forms chlorine and sulfurdioxide which again require special exhaust gas treatment.